Especially with inverters for feeding electric energy from photovoltaic devices, however, due to the huge total surface of the solar modules and due to the length of their cabling up to the respective inverter, there is an increased danger of transient overvoltages being coupled into the cabling of the strings by lightning strikes in the closer surroundings, i.e., by so-called nearby strikes, and being transferred onto the inverter. These transient overvoltages pose a danger to all electronic and electric parts of the inverter.
In order to protect inverters that feed electric energy from photovoltaic devices into AC power grids against transient overvoltages, it is common practice to connect surge arrestors to all lines running towards the inverter in such a way that the surge arrestors are effective both between the lines and between each individual line and ground. By means of these surge arrestors, the inverter is protected to a maximum extent, if they are provided directly before its DC input stage.
In an overvoltage protection apparatus comprising the features of the prior art, a DC input stage of the inverter includes an EMC Filter that includes interference suppressing capacitors and interference suppressing inductances. In combination with further measures, this EMC filter serves for electro-magnetic compliance when feeding electric energy via the inverter into the AC power grid.
Surge arrestors of the company DEHN+SÖHNE GmbH+Co. KG, Nuremberg, Germany are known by their product name DEHNguard®, in which two alternative current paths are provided. A varistor is provided in the one current path, a lead fuse in the other. A selector switch selecting between these two paths is at first switched to the one path with the varistor but is biased by a spring in the direction to the other path with the lead fuse. In this switching position it is held as long as a fusible component that is in thermal contact with the varistor is not yet fused. When the varistor is strongly loaded, such that its increase in temperature results in fusing of the fusible component, the selector switch is released and switches the ground current to the lead fuse. With a further continuing ground current, the lead fuse fuses. An electric arc that may occur here is extinguished by the lead fuse itself, which is designed accordingly and which is resistant against the influences of the electric arc. The selector switch does not only switch the ground current to the lead fuse but also moves a signalling pin leading outward out of the surge arrestor, by which it may be noticed optically or via a micro switch that the overvoltage protection has been stressed irreversibly and that at least a replacement of the fusible component and an inspection of the lead fuse are necessary. These known surge arrestors are complicated in their construction; they require special a self-extinguishing lead fuse; and they are accordingly expensive.
Usually, a DC voltage input stage of an inverter comprises a buffer capacitance of a considerable size in order to compensate for the pulsed current intake by the inverter and to smooth the backward influences from this, respectively.